The present invention relates to a capacitor, a process for the production of a capacitor, the capacitor obtainable by this process, an electronic circuit, the use of a capacitor and a dispersion.
A commercially available electrolyte capacitor as a rule is made of a porous metal electrode, an oxide layer serving as a dielectric on the metal surface, an electrically conductive material, usually a solid, which is introduced into the porous structure, an outer electrode (contacting), such as e.g. a silver layer, and further electrical contacts and an encapsulation. An electrolyte capacitor which is frequently used is the tantalum electrolyte capacitor, the anode electrode of which is made of the valve metal tantalum, on which a uniform, dielectric layer of tantalum pentoxide has been generated by anodic oxidation (also called “forming”). A liquid or solid electrolyte forms the cathode of the capacitor. Aluminium capacitors in which the anode electrode is made of the valve metal aluminium, on which a uniform, electrically insulating aluminium oxide layer is generated as the dielectric by anodic oxidation, are furthermore frequently employed. Here also, a liquid electrolyte or a solid electrolyte forms the cathode of the capacitor. The aluminium capacitors are usually constructed as wound- or stack-type capacitors.
π-conjugated polymers are particularly suitable as solid electrolytes in the capacitors described above because of their high electrical conductivity. π-conjugated polymers are also called conductive polymers or synthetic metals. They are increasingly gaining economic importance, since polymers have advantages over metals with respect to processability, weight and targeted adjustment of properties by chemical modification. Examples of known π-conjugated polymers are polypyrroles, polythiophenes, polyanilines, polyacetylenes, polyphenylenes and poly(p-phenylene-vinylenes), a particularly important polythiophene used industrially being poly(3,4-ethylenedioxythiophene) (PEDOT), since it has a very high conductivity in its oxidized form.
The solid electrolytes based on conductive polymers can be applied to the oxide layer in various ways and manners. EP-A-0 340 512 thus describes, for example, the preparation of a solid electrolyte from 3,4-ethylenedioxythiophene and the use thereof in electrolyte capacitors. According to the teaching of this publication, 3,4-ethylenedioxythiophene is polymerized on to the oxide layer in situ. After deposition of the polymer solid electrolyte, the oxide layer of the capacitor must conventionally be re-formed in order to achieve low residual currents, as is described, for example, in EP-A-0 899 757. For this, the capacitor is impregnated in an electrolyte and exposed to an electrical voltage which does not exceed the anodizing voltage of the oxide film.
The disadvantage of the production of solid electrolyte capacitors using an in situ polymerization is however, amongst others, the complexity of the process, and furthermore usually only unsatisfactorily high breakdown voltages are achieved.
Thus, a polymerization process which in each case includes the process steps of impregnation, polymerization and washing as a rule lasts several hours. Under certain circumstances, explosive or toxic solvents must also be employed here. A further disadvantage of the in situ process for the production of solid electrolyte capacitors is that as a rule anions of the oxidizing agent or, where appropriate, other monomeric anions serve as counter-ions for the conductive polymer. Because of their small size, however, these are not bonded to the polymer in a sufficiently stable manner. As a result, diffusion of the counter-ions and therefore an increase in the equivalent series resistance (ESR) of the capacitor may occur, especially at elevated use temperatures of the capacitor. The alternative use of high molecular weight polymeric counter-ions in the chemical in situ polymerization does not lead to sufficiently conductive films and therefore does not lead to low ESR values.
Alternative processes for the preparation of solid electrolytes based on conductive polymers in electrolyte capacitors have therefore been developed in the prior art. Thus, for example, DE-A-10 2005 043828 describes a process for the production of solid electrolytes in capacitors, in which a dispersion comprising the already polymerized thiophene, for example the PEDOT/PSS dispersions known from the prior art, is applied to the oxide layer and the dispersing agent is then removed by evaporation. The capacitors obtained from PEDOT/PSS dispersions have an increased breakdown voltage compared with those obtained by means of in situ polymerization.
However, there is a need to further increase the breakdown voltage, which is a measure of the reliability of an electrolyte capacitor, in particular for high use voltages. The breakdown voltage is the voltage at which the dielectric (oxide layer) of the capacitor no longer withstands the electrical field strength and electrical breakthroughs occur between the anode and cathode, which leads to a short circuit in the capacitor. The higher the breakdown voltage, the better the quality of the dielectric and the more reliable therefore also the capacitor. The nominal voltage at which the capacitor can be employed is also higher, the higher the breakdown voltage of the capacitor.
According to the teaching of WO-A-2007/097364, JP 2008-109065, JP 2008-109068 or JP 2008-109069, an increase in the breakdown voltage in aluminium capacitors can be achieved, for example, by adding polyethylene glycols to the polymer dispersions employed for producing the solid electrolyte layer before application of the dispersion to the oxide layer. The disadvantage of this set up, however, is that the long-term stability of the capacitor is limited at high temperatures, because the electrical properties, in particular the capacitance and the equivalent series resistance (ESR) of the capacitor, deteriorate over time at high temperatures. Such capacitors are therefore not suitable in particular for use in the automobile industry, and there in particular for use, for example, as intermediate capacitors (DC link capacitors) in hybrid and electric propulsion means, since exposure to particularly high temperatures occurs here. In these fields of use, the capacitors are employed in a temperature range of from +125° C. to +150° C. and a voltage range of from 100 V to 500 V. They must function reliably under these conditions and must have a very low equivalent series resistance (ESR), as is described, for example, by W. Wondrak et al., “Requirements on passive components for electric and hybrid vehicles”, CARTS Europe 2010, The 22nd Annual Passive Components Symposium, Proceedings, p. 34-39.
The use of polyglycerol in capacitors with layers of conductive polymers produced by chemical or electrochemical polymerization is known from US-A-2009/128997 and JP-A-2010-129864. The teaching of both specifications is that the long-term heat stability of capacitors at high temperatures can only be achieved if polyglycerol is employed in very small amounts in the chemical or electrochemical polymerization to obtain an outer layer of conductive polymer in a capacitor with several layers of conductive polymers.